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In terms of correlation statistics, this means that theoretically the correlation of tests scores between monozygotic twins would be 1.00 if genetics alone accounted for variation in IQ scores; likewise, siblings and dizygotic twins share on average half alleles and the correlation of their scores would be 0.50 if IQ were affected by genes ...
The power of twin designs arises from the fact that twins may be either identical (monozygotic (MZ), i.e. developing from a single fertilized egg and therefore sharing all of their polymorphic alleles) or fraternal (dizygotic (DZ), i.e. developing from two fertilized eggs and therefore sharing on average 50% of their alleles, the same level of genetic similarity found in non-twin siblings).
In 1979, Thomas J. Bouchard began to study twins who were separated at birth and reared in different families. He found that an identical twin reared away from his or her co-twin seems to have about an equal chance of being similar to the co-twin in terms of personality, interests, and attitudes as one who has been reared with his or her co ...
In the Minnesota Study of Twins Reared Apart, monozygotic twins separated shortly after birth were reunited in adulthood. [60] These adopted, reared-apart twins were as similar to one another as were twins reared together on a wide range of measures including general cognitive ability , personality , religious attitudes , and vocational ...
The effect of shared environment, c 2, contributes to similarity between siblings due to the commonality of the environment they are raised in. Shared environment is approximated by the DZ correlation minus half heritability, which is the degree to which DZ twins share the same genes, c 2 =DZ-1/2h 2.
One way to consider it is using trait X in twin 1 to predict trait Y in twin 2 for monozygotic and dizygotic twins (i.e. using twin 1's IQ to predict twin 2's brain volume); if this cross-correlation is larger for the more genetically-similar monozygotic twins than for the dizygotic twins, the similarity indicates that the traits are not ...
Identical twins reared apart from birth are another method of studying the origin of psychological traits. Unfortunately, such twin pairs are rare. Three sets of male twin pairs exist in the literature. In the first pair found by Thomas Bouchard, both male twins reared apart from birth were homosexual.
Burt was born on 3 March 1883, the first child of Cyril Cecil Barrow Burt (b. 1857), a medical practitioner, and his wife, Martha Decina Evans. [2] He was born in London (some sources give his place of birth as Stratford-upon-Avon, probably because his entry in Who's Who gave his father's address as Snitterfield, Stratford; in fact the Burt family moved to Snitterfield when he was ten).